[ios-sdk] Fixed build_framework.sh to create symlink correctly.
[ios-sdk] Fixed build_framework.sh to create symlink correctly.

Summary:
The symlink for Versions/Current was incorrectly pointing to "./Versions/A" rather
than to "./A"

Test Plan:
- Blew away 'build' directory
- Reran build
- Verified that 'FBiOSSDK.framework/Versions/Current' correctly dereferenced to 'A'

Reviewers: jacl, mmarucheck, vijaye, gregschechte

Reviewed By: mmarucheck

Differential Revision: https://phabricator.fb.com/D459275

Subsplash/facebook-ios-sdk,beamly/facebook-ios-sdk,flovilmart/facebook-ios-sdk,anthonylai/facebook-ios-sdk,TimeIncOSS/facebook-ios-sdk,1234-/facebook-ios-sdk,anthonylai/facebook-ios-sdk,ShareKit/facebook-ios-sdk,Citrrus/facebook-ios-sdk,anthonylai/facebook-ios-sdk,wooga/facebook-ios-sdk,wooga/facebook-ios-sdk,tipbit/facebook-ios-sdk,flovilmart/facebook-ios-sdk,1234-/facebook-ios-sdk,FiftyThree/facebook-ios-sdk,tipbit/facebook-ios-sdk,hibu/facebook-ios-sdk,ShareKit/facebook-ios-sdk,flovilmart/facebook-ios-sdk,hibu/facebook-ios-sdk,FiftyThree/facebook-ios-sdk,taptaptap/facebook-ios-sdk,Amnysia/facebook-ios-sdk,1234-/facebook-ios-sdk,beamly/facebook-ios-sdk,hibu/facebook-ios-sdk,Subsplash/facebook-ios-sdk,TimeIncOSS/facebook-ios-sdk,shepting/facebook-ios-sdk,Citrrus/facebook-ios-sdk,beamly/facebook-ios-sdk,flovilmart/facebook-ios-sdk,FiftyThree/facebook-ios-sdk,shepting/facebook-ios-sdk,ShareKit/facebook-ios-sdk,ArtLapitski/facebook-ios-sdk,tipbit/facebook-ios-sdk,TimeIncOSS/facebook-ios-sdk,wooga/facebook-ios-sdk,shepting/facebook-ios-sdk,flovilmart/facebook-ios-sdk,taptaptap/facebook-ios-sdk,shepting/facebook-ios-sdk,hibu/facebook-ios-sdk,Citrrus/facebook-ios-sdk,hibu/facebook-ios-sdk,Subsplash/facebook-ios-sdk,1234-/facebook-ios-sdk,ArtLapitski/facebook-ios-sdk,taptaptap/facebook-ios-sdk,ArtLapitski/facebook-ios-sdk,Amnysia/facebook-ios-sdk,Amnysia/facebook-ios-sdk,shepting/facebook-ios-sdk,ArtLapitski/facebook-ios-sdk,1234-/facebook-ios-sdk